For radio communications, there is a multi-input multi-output (MIMO) mode as a communication mode that realizes an improvement in communication speed. The MIMO mode is a technology for communications performed between communication devices provided with a plurality of antennas.
FIG. 5 is a diagram illustrating a configuration of a radio communication system 501 in the MIMO mode.
In the MIMO mode, a transmission-side communication device (transmission device) 502 divides transmission data into a plurality of data parts and distributes the generated data parts to a plurality of transmission antennas, thereby transmitting the data parts from the respective antennas with the same frequency. FIG. 5 illustrates an example of being provided with two transmission antennas 504 and 505.
A reception-side communication device (reception device) 503 receives signals transmitted from the transmission antennas 504 and 505 by a plurality of reception antennas (here, two reception antennas 506 and 507). From the received signals, the reception device 503 acquires the plurality of data parts transmitted from the respective transmission antennas by using a propagation path characteristic Hfwd exhibited in a direction from the transmission device 502 toward the reception device 503 between the transmission antennas 504 and 505 and the reception antennas 506 and 507, and generates reception data by combining the data parts.
Here, the propagation path characteristic Hfwd is expressed by a matrix having elements of propagation path characteristics h11, h12, h21, and h22 exhibited from the respective transmission antennas 504 and 505 toward the respective reception antennas 506 and 507.
The communications based on the MIMO mode can realize a communication speed obtained by being multiplied by the number of transmission antennas at maximum in comparison with a case of using one antenna.
In addition, there is an SVD-MIMO mode in which a perpendicular propagation path characteristic, which is obtained by subjecting the propagation path characteristic Hfwd to singular value decomposition (SVD) and is equivalent to the propagation path characteristic Hfwd, is used by the transmission device 502 and the reception device 503.
Described below is an outline of the SVD-MIMO mode in which signals are transmitted from the transmission device 502 provided with M (here, two) transmission antennas to the reception device provided with N (here, two) reception antennas.
In the communications based on the SVD-MIMO mode, first, the transmission device 502 transmits transmission-side known signals from the respective transmission antennas 504 and 505 to the respective reception antennas 506 and 507.
The reception device 503 uses transmission-side known signals that are previously grasped and the transmission-side known signals that have been transmitted from the transmission antennas 504 and 505 and received by the reception antennas 506 and 507 to calculate the propagation path characteristic Hfwd exhibited in the direction from the transmission device 502 toward the reception device 503. The propagation path characteristic Hfwd is a radio channel matrix of N rows and M columns.
Subsequently, the reception device 503 subjects the propagation path characteristic Hfwd to the singular value decomposition as shown in Expression (1).
                    [                  Numeral          ⁢                                          ⁢          1                ]                                                                                  H            fwd                    =                      U            ⁢                          ∑                              V                H                                                    ⁢                                  ⁢                  ∑                      =                          [                                                                                                                  λ                        1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        λ                        2                                                                                                                                                                                                    0                                                                                                                                                                                                                                                                                            ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                                          λ                        m                                                                                                        ]                                                          (        1        )            
Here, “U” represents a unitary matrix of M rows and M columns, “V” represents a unitary matrix of N rows and N columns, and “H” appended to “V” represents a conjugate transpose.
The reception device 503 performs a feedback of the obtained unitary matrix V to the transmission device 502.
The transmission device 502 performs weighting by multiplying a vector s formed of M (that is, two) transmission signals by the unitary matrix V to generate a signal Vs, and transmits the obtained signal Vs from the transmission antennas 504 and 505.
The reception device 503 receives a reception signal vector y, which is formed of N (that is, two) signals on which the respective elements of Vs transmitted from the transmission antennas 504 and 505 are superposed, by the reception antennas 506 and 507. The reception device 503 performs demodulation by multiplying the signals y by the conjugate transpose UH of the unitary matrix U obtained by the singular value decomposition shown in Expression (1).
In this manner, between the plurality of transmission antennas 504 and 505 and the plurality of reception antennas 506 and 507, exchanges of data items having no correlation therebetween are realized.
This mode is referred to as “closed loop SVD-MIMO mode” because the reception device 503 performs a feedback of the unitary matrix V to the transmission device 502, and is disclosed in, for example, Non Patent Literature 1.